An index-title-scrubber-bar, for use with a UICollectionView (or even a PSTCollectionView). Gives a collection view the index title bar that a UITableView gets for (almost) free. A huge thank you to @yang from http://stackoverflow.com/a/14443540/194869, which saved my bacon here.

BDKCollectionIndexView.m

/*
 *  BDKCollectionIndexView.h
 */

#import <UIKit/UIKit.h>

/** The direction in which the control is oriented. Assists in determining layout values.
 */
typedef enum {
    BDKCollectionIndexViewDirectionVertical = 0,
    BDKCollectionIndexViewDirectionHorizontal
} BDKCollectionIndexViewDirection;

/** An index-title-scrubber-bar, for use with a UICollectionView (or even a PSTCollectionView). Gives a collection
 *  view the index title bar that a UITableView gets for (almost) free. A huge thank you to
 *  @Yang from http://stackoverflow.com/a/14443540/194869, which saved my bacon here.
 */
@interface BDKCollectionIndexView : UIControl

/** A collection of string values that represent section index titles.
 */
@property (strong, nonatomic) NSArray *indexTitles;

/** Indicates the position of the last-selected index title. Should map directly to a table view / collection section.
 */
@property (readonly, nonatomic) NSUInteger currentIndex;

/** The number of points used for padding at the end caps of the view (assists in layout).
 */
@property (nonatomic) CGFloat endPadding;

/** The direction in which the control is oriented; this is automatically set based on the frame given.
 */
@property (readonly) BDKCollectionIndexViewDirection direction;

/** The index title at the index of `currentIndex`.
 */
@property (readonly) NSString *currentIndexTitle;

/** A class message to initialize and return an index view control, given a frame and a list of index titles.
 *  @param frame the frame to use when initializing the control.
 *  @param indexTitles the index titles to be rendered out in the control.
 *  @return an instance of the class.
 */
+ (id)indexViewWithFrame:(CGRect)frame indexTitles:(NSArray *)indexTitles;

/** A message to initialize and return an index view control, given a frame and a list of index titles.
 *  @param frame the frame to use when initializing the control.
 *  @param indexTitles the index titles to be rendered out in the control.
 *  @return an instance of the class.
 */
- (id)initWithFrame:(CGRect)frame indexTitles:(NSArray *)indexTitles;

@end


/*
 *  BDKCollectionIndexView.m
 */

#import "BDKCollectionIndexView.h"
#import <QuartzCore/QuartzCore.h>

@interface BDKCollectionIndexView ()

/** A component that shows up under the letters to indicate the view is handling a touch or a pan.
 */
@property (strong, nonatomic) UIView *touchStatusView;

/** The collection of label subviews that are displayed (one for each index title).
 */
@property (strong, nonatomic) NSArray *indexLabels;

/** A gesture recognizer that handles panning.
 */
@property (strong, nonatomic) UIPanGestureRecognizer *panner;

/** A gesture recognizer that handles tapping.
 */
@property (strong, nonatomic) UITapGestureRecognizer *tapper;

/** A gesture recognizer that handles panning.
 */
@property (readonly) CGFloat theDimension;

/** Handles events sent by the tap gesture recognizer.
 *  @param recognizer the sender of the event; usually a UIPanGestureRecognizer.
 */
- (void)handleTap:(UITapGestureRecognizer *)recognizer;

/** Handles events sent by the pan gesture recognizer.
 *  @param recognizer the sender of the event; usually a UIPanGestureRecognizer.
 */
- (void)handlePan:(UIPanGestureRecognizer *)recognizer;

/** Handles logic for determining which label is under a given touch point, and sets `currentIndex` accordingly.
 *  @param point the touch point.
 */
- (void)setNewIndexForPoint:(CGPoint)point;

/** Handles setting the alpha component level for the background color on the `touchStatusView`.
 *  @param flag if `YES`, the `touchStatusView` is set to be visible and dark-ish.
 */
- (void)setBackgroundVisibility:(BOOL)flag;

@end

@implementation BDKCollectionIndexView

@synthesize currentIndex = _currentIndex, direction = _direction, theDimension = _theDimension;

+ (id)indexViewWithFrame:(CGRect)frame indexTitles:(NSArray *)indexTitles {
    return [[self alloc] initWithFrame:frame indexTitles:indexTitles];
}

- (id)initWithFrame:(CGRect)frame indexTitles:(NSArray *)indexTitles {
    if (self = [super initWithFrame:frame]) {
        if (CGRectGetWidth(frame) > CGRectGetHeight(frame))
            _direction = BDKCollectionIndexViewDirectionHorizontal;
        else _direction = BDKCollectionIndexViewDirectionVertical;
        
        _currentIndex = 0;
        _endPadding = 2;
        
        _panner = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(handlePan:)];
        [self addGestureRecognizer:_panner];
        _tapper = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(handleTap:)];
        [self addGestureRecognizer:_tapper];
        
        [self addSubview:self.touchStatusView];
        
        self.indexTitles = indexTitles;
    }
    
    return self;
}

- (void)layoutSubviews {
    
    CGFloat maxLength = 0.0;
    switch (_direction) {
        case BDKCollectionIndexViewDirectionHorizontal:
            _theDimension = CGRectGetHeight(self.frame);
            maxLength = CGRectGetWidth(self.frame) - (self.endPadding * 2);
            break;
        case BDKCollectionIndexViewDirectionVertical:
            _theDimension = CGRectGetWidth(self.frame);
            maxLength = CGRectGetHeight(self.frame) - (self.endPadding * 2);
            break;
    }
    
    self.touchStatusView.frame = CGRectInset(self.bounds, 2, 2);
    self.touchStatusView.layer.cornerRadius = floorf(self.theDimension / 2.75);
    
    CGFloat cumulativeLength = self.endPadding;
    CGSize labelSize = CGSizeMake(self.theDimension, self.theDimension);
    
    CGFloat otherDimension = floorf(maxLength / self.indexLabels.count);
    for (UILabel *label in self.indexLabels) {
        switch (self.direction) {
            case BDKCollectionIndexViewDirectionHorizontal:
                labelSize.width = otherDimension;
                label.frame = (CGRect){ { cumulativeLength, 0 }, labelSize };
                cumulativeLength += CGRectGetWidth(label.frame);
                break;
            case BDKCollectionIndexViewDirectionVertical:
                labelSize.height = otherDimension;
                label.frame = (CGRect){ { 0, cumulativeLength }, labelSize };
                cumulativeLength += CGRectGetHeight(label.frame);
                break;
        }
    }
}

#pragma mark - Properties

- (UIView *)touchStatusView {
    if (_touchStatusView) return _touchStatusView;
    _touchStatusView = [[UIView alloc] initWithFrame:CGRectInset(self.bounds, 2, 2)];
    _touchStatusView.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0];
    _touchStatusView.layer.cornerRadius = self.theDimension / 2;
    _touchStatusView.layer.masksToBounds = YES;
    return _touchStatusView;
}

- (void)setIndexTitles:(NSArray *)indexTitles {
    if (_indexTitles == indexTitles) return;
    _indexTitles = indexTitles;
    [self.indexLabels makeObjectsPerformSelector:@selector(removeFromSuperview)];
    [self buildIndexLabels];
}

- (NSString *)currentIndexTitle {
    return self.indexTitles[self.currentIndex];
}

- (void)setEndPadding:(CGFloat)endPadding {
    if (_endPadding == endPadding) return;
    _endPadding = endPadding;
    
    [self.indexTitles makeObjectsPerformSelector:@selector(removeFromSuperview)];
    [self buildIndexLabels];
}

#pragma mark - Subviews

- (void)buildIndexLabels {
    
    NSMutableArray *workingLabels = [NSMutableArray arrayWithCapacity:self.indexTitles.count];
    for (NSString *indexTitle in self.indexTitles) {
        UILabel *label = [[UILabel alloc] initWithFrame:CGRectZero];
        label.text = indexTitle;
        label.font = [UIFont boldSystemFontOfSize:12];
        label.backgroundColor = [UIColor clearColor];
        label.textColor = [UIColor colorWithRed:0.415 green:0.451 blue:0.490 alpha:1.0];
        label.textAlignment = NSTextAlignmentCenter;
        [self addSubview:label];
        [workingLabels addObject:label];
    }
    
    self.indexLabels = [NSArray arrayWithArray:workingLabels];
}

- (void)setNewIndexForPoint:(CGPoint)point {
    for (UILabel *view in self.indexLabels) {
        if (CGRectContainsPoint(view.frame, point)) {
            NSUInteger newIndex = [self.indexTitles indexOfObject:view.text];
            if (newIndex != _currentIndex) { 
                _currentIndex = newIndex;
                [self sendActionsForControlEvents:UIControlEventValueChanged];
            }
        }
    }
}

- (void)setBackgroundVisibility:(BOOL)flag {
    CGFloat alpha = flag ? 0.25 : 0;
    self.touchStatusView.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:alpha];
}

#pragma mark - Gestures

- (void)handleTap:(UITapGestureRecognizer *)recognizer {
    [self setBackgroundVisibility:!(recognizer.state == UIGestureRecognizerStateEnded)];
    [self setNewIndexForPoint:[recognizer locationInView:self]];
}

- (void)handlePan:(UIPanGestureRecognizer *)recognizer {
    [self setBackgroundVisibility:!(recognizer.state == UIGestureRecognizerStateEnded)];
    CGPoint translation = [recognizer locationInView:self]; 
    [self setNewIndexForPoint:translation];
}

@end